The first part is correct but it can be shortened in working in a diagram form alternatively.
B R G
3 <–5units–> (x 3 to make them equal = 15 units)
x3 =9u 7u 8u (total = 24u)
So 9u-7u = 32 => u = 16
24u =384, 8u = 128
Alternative Solution for part 2:
Let p be the added green pens
(128+p)/(383+p) = 68/100
12800 + 100p = 26112 + 68p
32p =13312 => p =416
Hence number of green pens added is 416.